U.S. patent number 4,517,645 [Application Number 06/364,402] was granted by the patent office on 1985-05-14 for control device for loading and unloading mechanism.
This patent grant is currently assigned to Kabushiki Kaisha Meidensha, Kabushiki Kaisha Toyoda Jidoh Shokki Seisakusho. Invention is credited to Masaru Kawamata, Yasuyuki Miyazaki, Mineo Ozeki, Susumu Yoshida, Katsumi Yuki.
United States Patent |
4,517,645 |
Yuki , et al. |
May 14, 1985 |
Control device for loading and unloading mechanism
Abstract
A control device for loading and unloading mechanism adapted to
be incorporated in a fork lift truck comprises a sensor unit 100
including a lifting height sensor 102 and a load sensor 106, a
control unit 200 comprising a control command producing circuit 240
constituted by a microcomputer 230 producing a control command on
the basis of comparing calculation between the output of the sensor
unit 100 and the concerned data stored in the microcomputer 230, a
servomotor driving circuit 322' responsive to the control command
indicative of a valve opening angle, and a hydraulic pressure
driving circuit 340 for actuating a lift cylinder in accordance
with the output of the servomotor driving circuit 322'. The control
device is constituted so that, when the work for piling or
unloading a load on a shelf is effected based on the stored lifting
height data, the two positions required for piling and unloading
can be stored in the same address allotted to the corresponding
shelf can be stored. The control device is constituted so that,
when lifting height data is stored in the microcomputer, an
indication for confirming data storage is added to the stored
lifting height data, thereby enabling to prevent an automatic
lifting height control from being erroneously effected.
Inventors: |
Yuki; Katsumi (Toyota,
JP), Yoshida; Susumu (Aichi, JP), Ozeki;
Mineo (Ichinomiya, JP), Miyazaki; Yasuyuki
(Aichi, JP), Kawamata; Masaru (Numazu,
JP) |
Assignee: |
Kabushiki Kaisha Toyoda Jidoh
Shokki Seisakusho (both of, JP)
Kabushiki Kaisha Meidensha (both of, JP)
|
Family
ID: |
26387908 |
Appl.
No.: |
06/364,402 |
Filed: |
March 31, 1982 |
Foreign Application Priority Data
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Mar 31, 1981 [JP] |
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56-47739 |
Mar 31, 1981 [JP] |
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56-47743 |
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Current U.S.
Class: |
701/50; 414/273;
414/636 |
Current CPC
Class: |
B66F
9/0755 (20130101); B66F 9/24 (20130101); G05B
19/4142 (20130101); G05B 19/23 (20130101); G05B
2219/41309 (20130101); G05B 2219/34215 (20130101) |
Current International
Class: |
B66F
9/075 (20060101); B66F 9/24 (20060101); G05B
19/23 (20060101); G05B 19/414 (20060101); G05B
19/19 (20060101); G06F 015/50 (); B66F
009/06 () |
Field of
Search: |
;364/424,447,559,562
;187/29R,29A,29B ;340/686 ;414/272-275,632-638,674 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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53-20263 |
|
Feb 1978 |
|
JP |
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54-37378 |
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Nov 1979 |
|
JP |
|
Primary Examiner: Chin; Gary
Attorney, Agent or Firm: Lowe, King, Price & Becker
Claims
What is claimed is:
1. In a control device for a loading and unloading mechanism
adapted to a fork lift truck comprising:
(a) a sensor unit (100) including at least a lifting height sensor
means (102) for measuring a lifting height of a fork movably
mounted on the fork lift truck, and a load sensor means (106) for
detecting the weight of a load supported by the fork,
(b) a control unit (200) comprising an interface circuit including
a lifting height counter means (222) for counting output signals
from the sensor unit, and first data setting means manually
operable by an operator,
(c) a control command producing circuit (240) provided in the
control unit (200), said control command producing circuit (240)
including a memory means (244) for storing data indicative of
lifting height, and second data setting means (246) manually
operable by an operator for setting data indicative of lifting
height into the memory means, said control command producing
circuit producing a command signal indicative of valve opening on
the basis of a comparison between outputs of the sensor unit (100)
and the data stored in the memory means,
(d) a servomotor driving circuit means (320) responsive to the
command signal indicative of valve opening produced by the control
unit to produce a drive control signal, and
(e) a hydraulic pressure driving circuit means (340) responsive to
the driving control signal for producing a control signal for
hydraulically controlling a lift cylinder (346),
the improvement wherein
said control command producing circuit comprises storage control
means for storing in said memory means data indicative of a
predetermined allowable lifting height control range input to the
memory means by said second data setting means prior to a loading
and unloading operation, and
said storage control means further operable for storing in a first
memory area a first lifting height data indicative of a preselected
lower value (H.sub.l), required for a loading and unloading
operation and a second lifting height data indicative of a
preselected higher value (H.sub.h) required for the same operation
in a memory location corresponding to a common address designated
by said second data setting means,
wherein said storage control means is further operable for storing
in a second memory area storage indication data for confirming
storage in said first memory area of data indicative of a lifting
height, said second memory area allotted to a predetermined bit or
bits of a memory area outside of the first memory area in which the
lifting height data are stored.
2. A control device for a loading and unloading mechanism as
defined in claim 1, wherein said storage control means is operable
for storing said storage indication data confirming storage of said
lifting height data in a memory location in said second memory area
having an address related to the address of a memory location in
the first memory area in which the corresponding lifting height
data are stored.
3. A control device for a loading and unloading mechanism as
defined in claim 1 wherein said data indicative of a predetermined
allowable lifting height control range are stored in the memory
means by said second data setting means prior to the loading and
unloading operation, and said control command producing circuit 240
comprises means for inhibiting storage of lifting height data in
the memory means 244 when the actual lifting height data sensed by
the lifting height sensor means 102 exceeds a preselected upper
limit, or is equal to or less than a preselected lower limit.
4. A control device for a loading and unloading mechanism as
defined in claim 3, wherein said means for inhibiting comprises a
first up-down counter 222A responsive to the output of the lifting
height sensor means 102, a second up-down counter 222B for
presetting said upper limit, responsive to an output signal of said
first up-down counter 222A, and a third up-down counter 222C for
presetting said lower limit, responsive to said first up-down
counter 222A, whereby storage of the lifting height data in the
memory 224 is inhibited by the counter of either of said second and
third up-down counters 222B, 222C.
5. A control device for a loading and unloading mechanism as
defined in claim 1 wherein the control unit 200 further comprises a
control circuit means 260 responsive to a difference between the
command signal from said control command producing circuit 240 and
a feedback signal of the servomotor driving circuit 320 for
controlling the servomotor driving circuit means 320.
6. A control device for a loading and unloading mechanism as
defined in claim 5 wherein said control command producing circuit
240 provides for said command signal a plurality of discrete levels
on the basis of a signal derived from said lifting height sensor
means 102 representing lifting speed.
7. A control device for a loading and unloading mechanism as
defined in claim 6, wherein said control command producing circuit
240 compares the lifting height data previously set into said
memory means with the actual lifting height indicated by an output
of a lifting height counter means 222 connected for counting the
output of said lifting height sensor means 102 in a predetermined
time interval after receipt of said output, and thereafter provides
said plurality of discrete levels for said command signal.
8. A control device for a loading and unloading mechanism as
defined in claim 5 wherein a data table representing a relationship
between preset lifting speed data and the absolute value of a
difference between a desired lifting height and a present lifting
height is stored in said memory means, and wherein said control
command producing circuit 240 is operable for producing a command
signal in accordance with the difference between the preset lifting
height data read from said data pattern and the present lifting
height.
9. A control device for a loading and unloading mechanism as
defined in claim 8 wherein said control command producing circuit
further comprises timer means for measuring time intervals between
pulses of a pulse output signal from said lifting height sensor
means 102, thereby obtaining lifting speed.
10. A control device for a loading and unloading mechanism as
defined in claim 5 wherein there is provided a push-button switch
232B activated for slowly stopping the hydraulic pressure driving
circuit means, said control command producing circuit 240 producing
a decelerating stepping command related to the speed of the fork
immediately before the push-button switch 232B is actuated on the
basis of one of a predetermined time and a predetermined
distance.
11. A control device for a loading and unloading mechanism as
defined in claim 7 wherein said control command producing circuit
240 produces a signal for limiting a valve opening angle of a
control valve of said hydraulic pressure driving circuit means to a
predetermined region.
12. A control device for a loading and unloading mechanism as
defined in claim 1 wherein said storage control means is further
operable for particularly storing said first and second lift height
data at a memory location having a common address related to an
individual shelf associated with said data.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a control device for a loading and
unloading mechanism, and more particularly to a lift cylinder
control applied to a fork lift truck. Specifically, the present
invention is concerned with a control device for a loading and
unloading mechanism wherein the system control for governing the
operation of a lift cylinder is supervised by a microcomputer.
As is well known, a fork lift truck comprises a loading and
unloading mechanism and a vehicle body. The loading and unloading
mechanism comprises a vertically elongated guide rail called an
"upright", and a fork slidable in the upright. The mechanism
further comprises a hydraulic member, as for example, hydraulic
cylinder for lifting and lowering the fork and tilting the
upright.
In connection with the prior art loading and unloading control, for
instance, lifting height control, drawbacks are pointed out as
follows: Recently, there is a tendency that the lifting height
becomes high when loading and unloading work is effected with a
fork lift truck. For instance, the piling and unloading may be
effected at heights greater than 10 m. In such a case, it is
difficult for an operator to adjust the loading and unloading
mechanism so that the fork is placed at the predetermined height,
looking at the top of the fork positioned above about 10 m
relatively to the seat of the operator. Accordingly, it is
desirable for the operator to easily effect piling and unloading
the load at the predetermined position.
In order to embody this requirement in the prior art, the upright
is provided with a limit switch for stopping the fork at a
predetermined position. When the fork reaches the predetermined
position, for instance, 8.5 m, the control device is designed so as
to light a lamp provided at the operator's unit or break a driving
power supply for loading and unloading operations. Usually, a load
is unloaded on a shelf with a plurality of steps. For this reason,
in order to determine the desired position it is required to select
the step. The provision of a predetermined number of limit
switches, for instance ten, is required in order to meet the height
of the shelf. Further, it happens that the piling and unloading is
required at another shelf according to the change of the working
place. In such a case, if the height of the shelf is different from
that of the prior one, a more complicated control device is
required. Actually, it has been impossible to effect the piling and
unloading operation. Further, from the point of view of the system
control in the prior art, a plurality of analog control circuits,
such as, comprising combination of relay circuits respectively
provided with respect to the controlled system, as for example,
lifting height control are incorporated in the control unit of the
control device for loading and unloading mechanism. Prior to the
lifting work, an operator effects various settings according to the
lifting height condition required for loading and unloading
operation and then starts a lifting height work. In this instance,
an automatic control system is constituted, which includes therein
a valve opening control system provided with respect to a hydraulic
pressure circuit for actuating a lift cylinder. The lifting height
control is effected so as to control the valve opening control
system due to the deviation between an actual lifting height above
said setting value. However, when the setting is changed to a great
extent according to the change of the loading and unloading working
place, it is required to adjust the automatic control system in
order to stabilize the control system. Alternately, it happens that
the desired control accuracy cannot be obtained. Further, such a
lifting height control is effected in a series of sequential
control for loading and unloading work with the lifting height
control being related to various kinds of controls. Accordingly, it
is desirable to supervise the whole system control in view of the
simplicity of the circuit and harmonious execution of the
control.
In view of this, another attempt has been made. The programmed
series of sequential control matching with the objective loading
and unloading operation is stored in a computer, such as a
microcomputer. When, for instance, lifting height control is
effected, the concerned programmed routine for lifting height
control is called from the program to effect a lifting height
control due to the execution of the programmed routine.
In the prior art, when a load is piled or unloaded on a shelf with
a fork lift truck into which a computer controlled device is
incorporated, drawbacks are pointed as follows: The method of
piling a load on a shelf comprises the steps of moving an unloaded
fork lift truck near the shelf, lifting the unloaded fork to the
predetermined shelf, running the fork lift truck at that position
in the forward direction, mounting the load on the fork, lifting
the fork within the same shelf by the predetermined height, running
the fork lift truck at that position in the backward direction, and
lowering the fork to the predetermined running position. On the
other hand, the method of unloading a load from a shelf comprises
the steps of moving the loaded fork lift truck near the shelf,
lifting the fork to the predetermined shelf, running the fork lift
truck at that position in the forward direction, lowering the
loaded fork within the same shelf by the predetermined height,
running the fork lift truck at that position in the backward
direction, mounting the load on the shelf, and lowering the fork to
the predetermined running position.
When the piling and unloading is effected with the above-mentioned
fork lift truck, the desired shelf is selected by actuating an
address selecting key switch corresponding to each shelf position
provided on an operating box incorporated in the truck body. In
this instance, the designation or selection of two height positions
within the same shelf is required. Accordingly, the actuation of
the address selection key switch is troublesome.
Further, an automatic loading and unloading control, such as,
lifting height control is effected in accordance with the
sequential control matching with the objective loading and
unloading work. An operator actuates the address selecting key
switch in order to designate the address allotted to the lifting
height value. In this instance, if the address to which a desired
lifting height data is assigned is erroneously designated, the
automatic control is effected under the condition that the data
stored in this address is selected as a lifting height objective
value. This brings about a serious safety problem.
In an automatic loading and unloading control supervised by a
microcomputer, in addition to the drawbacks stated above, when a
lifting height control is actually effected in accordance with the
sequential data in connection with a loading and unloading
operation stored in the microcomputer, the following drawbacks are
further pointed out: When the lifting speed of the fork is
controlled by an automatic lifting height control effected due to
the stored lifting height, if a command for changing speed is
given, it has been difficult to effect a follow-up control because
of the fact that the characteristic of the opening angle of the
lift valve with respect to the lifting or lowering speed of the
fork in non-linear, and that there exists a response delay inherent
in the automatic control system. Furthermore, when the fork reaches
the objective lifting height and then is stopped thereat, there is
not provided a mechanism for slowly stopping the fork. Accordingly,
the fork is stopped suddenly, which brings about a safety
problem.
SUMMARY OF THE INVENTION
With the above in mind, an object of the present invention is to
provide a control device for loading and unloading mechanism making
it possible to improve the operation of an automatic control of a
fork effected in accordance with the stored sequential data for
loading and unloading work.
Another object of the invention is to provide a control device for
a loading and unloading mechanism making it possible to store the
two positions required for piling and unloading in the same address
allotted to a corresponding shelf, when the loading and unloading
operation, such as the piling or unloading a load on a shelf, is
effected in accordance with, stored lifting height data.
Another object of the invention is to provide a control device for
a loading and unloading mechanism wherein when lifting height data
are stored, for example, the indication for confirming data storage
is entered into a vacant bit or bits other than bits used for
storing lifting height data, or into a memory area allotted to an
address related to the above-mentioned address thereby making it
possible to prevent an automatic lifting height control from being
erroneously effected.
Another object of the invention is to provide a control device for
a loading and unloading mechanism capable of effecting a harmonious
follow-up control of a fork to the objective value at the time of
an automatic lifting height control.
Another object of the invention is to provide a control device for
a loading and unloading mechanism making it possible to gradually
approach the objective value due to a response delay of an
automatic control system or a lifting height speed control when an
automatic lifting height control is effected, thereby enabling to
slowly and securely stop a fork at the objective value.
Another object of the present invention is to provide a control
device for a loading and unloading mechanism wherein there is
provided a slow stopping means in a command producing circuit, e.g.
microcomputer, thereby enabling to slowly stop a fork at the
objective value to improve safety.
Another object of the present invention is to provide a control
device for a loading and unloading mechanism making it possible to
sample lifting height data within a predetermined range when
lifting height data is stored in a command producing circuit, e.g.
microcomputer, thereby enabling the control device to effect a
harmonious automatic lifting height control.
Another object of the invention is to provide a control device for
a loading and unloading mechanism wherein a command indicative of a
valve opening angle for actuating a lift cylinder which lifts and
lowers a fork is limited to a predetermined range, thereby
stabilizing a lifting height speed when an automatic lifting height
control is effected.
According to the present invention, there is provided a control
device for a loading and unloading mechanism adapted to be
incorporated in a fork lift truck comprising: a sensor unit, a
control unit responsive to the output signal of the sensor unit,
the control unit effecting a calculation on the basis of the output
signal thereform and producing a predetermined control signal
according to the calculated value, a servomotor driving circuit
responsive to the output signal of the control unit, and a
hydraulic pressure driving circuit for lifting and lowering a fork
responsive to the predetermined control signal of the servomotor
driving circuit, the hydraulic pressure driving circuit producing a
driving output signal for adjusting the valve opening angle for
actuating a lift cylinder, characterized in that the control unit
comprises an interface circuit for inputting the output signal from
the sensor unit, and a control command producing circuit comprising
a memory for storing a predetermined lifting height data and a data
setting means for setting a data to the memory, and in that the
control command producing circuit produces a control command on the
basis of comparing calculation between the output of the sensor
unit and the concerned data stored in the memory to effect a
desired lifting height control in accordance with the control
command.
BRIEF DESCRIPTION OF THE DRAWINGS
The feature and advantages of a control device for loading and
unloading mechanism according to the present invention will become
more apparent from the description taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a block diagram schematically illustrating a system
construction of a control device for a loading and unloading
mechanism according to the present invention;
FIG. 2 is a side view illustrating a fork lift truck to which the
present invention is applied;
FIG. 3 is a side view illustrating a lifting height sensor
incorporated in the fork lift truck shown in FIG. 2;
FIG. 4 is a block diagram illustrating a first embodiment of a
control device for a loading and unloading mechanism according to
the present invention;
FIGS. 5A and 5B are views for explaining loading and unloading
operations effected with a fork lift truck into which the control
device shown in FIG. 4 is incorporated;
FIG. 6 is a flow chart showing a procedure for storing lifting
height data employed in the first embodiment of the invention;
FIG. 7 is a flow chart showing a procedure for controlling the
lifting height employed in the first embodiment of the
invention;
FIG. 8 is a block diagram illustrating a second embodiment of a
control device for loading and unloading mechanism according to the
present invention;
FIG. 9 is a view for explaining a condition wherein a lifting
height storing indication is alloted to vacant bits in a memory
area in which lifting height data is stored in accordance with the
second embodiment of the invention;
FIG. 10 is a flow chart showing that program execution is shifted
to an automatic lifting height control upon the confirmation of the
lifting height sorting indication;
FIG. 11 is a side view for explaining a lifting height operation
according to the third embodiment of the invention;
FIG. 12 is a flow chart for checking lifting height data stored in
a microcomputer with respect to the stored upper and lower limits
in accordance with the third embodiment of the invention;
FIG. 13 is a block diagram illustrating a checking circuit for
embodying the function indicated by the flow chart of FIG. 12;
FIG. 14 is a block diagram illustrating a conventional lifting
height control device for a loading and unloading mechanism;
FIG. 15 is a block diagram illustrating a fourth embodiment of a
control device for a loading and unloading mechanism according to
the present invention;
FIG. 16 is a flow chart for effecting a lifting height control with
the control device shown in FIG. 15,
FIG. 17 illustrates a speed characteristic curve of a fork when a
lifting height control is effected with the control device shown in
FIG. 15,
FIG. 18 is a graph illustrating a valve opening angle setting
signal with respect to a command signal fed from a microcomputer
employed in the control device shown in FIG. 15,
FIG. 19 is a flow chart showing an automatic speed control
immediately before the objective height effected by a fifth
embodiment of the control device for a loading and unloading
mechanism according to the present invention;
FIGS. 20A and 20B are waveforms illustrating a sensor pulse train
and a timer pulse train, respectively, which are used at a step
four of the FIG. 19 flow chart;
FIG. 20C is a flow chart for producing the timer pulse train shown
in FIG. 20B;
FIG. 21 is a flow chart illustrating a main program for automatic
lifting height control employed in a sixth embodiment according to
the present invention;
FIG. 22 is a flow chart illustrating a subroutine for slow stop
interrupting command employed in the sixth embodiment according to
the invention,
FIGS. 23A and 23B are views for explaining a lifting height
operation effected with the control device of the sixth embodiment
according to the invention;
FIGS. 24 and 25 are graphs each illustrating the relationship
between lifting speed and valve opening angle in the sixth
embodiment of the present invention; and
FIG. 26 is a flow chart illustrating an automatic lifting height
speed control routine employed in the seventh embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a block diagram illustrating a system construction of a
control device for loading and unloading mechanism according to the
present invention.
Reference numeral 100 denotes a sensor unit including a lifting
height sensor 102, a tilting angle sensor 104, and a load sensor
106 (hydraulic pressure sensor). Reference numeral 200 denotes a
control unit comprises an interface circuit 220 including a lifting
height counter 222, a control command producing circuit 240
constituted by a microcomputer 230 responsive to the output of the
sensor unit 100 fed through the interface circuit 220, and a
control circuit 260 responsive to the control command being output
from the control command producing circuit 240. Reference numerals
110S and 112S denote contacts for manual setting, which are closed
by external commands indicative of lifting height and the
horizontal position of the fork, respectively.
More particularly, the control command producing circuit 240
comprises a central processing unit (CPU) designated by reference
numeral 242, a memory 244 essentially consisting of a random access
memory (RAM) designated by reference numeral 244A, a read only
memory (ROM) designated by reference numeral 244B in which
predetermined lifting height, tilting angle, load, or other data
are stored, and a data setting means 246, as for example,
comprising a key board for setting desired data by an operator. The
control command producing unit 240 produces a control command based
on the output of the sensor unit 100 and the data in connection
with lifting height, tilting angle, or load stored in the memory
244. The control circuit 260 comprises a first control circuit 262
for lifting height control system and a second control circuit 264
for tilting angle control system.
Reference numeral 300 denotes a driving unit comprising an
electric/hydraulic pressure converter 320 and a hydraulic pressure
driving unit 340. The electric/hydraulic pressure convertor 320
comprises a first and a second actuators 322 and 324 responsive to
the output of the first and second control circuits 262 and 264,
respectively. The first actuator 322 comprises a servomotor driving
circuit (referred to later) essentially consisting of switching
transistors 322T.sub.1 to 322T.sub.4 constituting an inverter for
controlling a driving motor 322M, and a contact 322S for connecting
a DC power supply 322B to the inverter on the basis of the command
fed from the first control circuit 262, and a link mechanism (not
shown) for joining the output shaft (not shown) of the driving
motor 322M to a lift valve member referred to soon. Likewise, the
second actuator 324 comprises a servomotor driving circuit
(referred to later) essentially consisting of switching transistors
324T.sub.1 to 324T.sub.4 constituting an inverter for controlling a
driving motor 324M, and a contact 324S for connecting a DC power
supply 324B to the inverter on the basis of the command fed from
the second control circuit 264, and a link mechanism (not shown)
for joining the output shaft (not shown) of the driving motor 324M
to a tilt valve member referred to soon. The hydraulic pressure
driving unit 340 comprises a first and a second control valves
responsive to the first and the second actuators 322 and 324,
respectively. The first control valve 342 is connected to a lift
cylinder 346 for controlling a lifting height, while the second
control valve 344 is connected to a tilting cylinder 348 for
controlling a tilting angle. Between the first and second control
valves 342 and 344, there is provided a hydraulic pump 345P for
supplying a suitable hydraulic oil thereto. Reference numeral 345T
denotes a hydraulic oil tank. Reference numeral 345S denotes a
contact provided in an electromagnetic valve (not shown) for
feeding and interrupting a hydraulic oil fed from the hydraulic
pump 345P in accordance with an external command. The
above-mentioned first control circuit 262, the first actuator 322,
first control valve 342, and a lift cylinder 346 constitute a servo
control circuit for lifting height control system. Likewise, the
above-mentioned second control circuit 264, the second actuator
324, and the second control valve 344, and a tilt cylinder 348
constitute a servo control circuit for tilting angle control
system.
FIG. 2 shows a fork lift truck to which the control device for
loading and unloading mechanism according to the present invention
is applied. Reference numeral 10 denotes a pair of uprights
provided on the right and left sides, each comprising an outer mast
10A and an inner mast 10B supported by the outer mast 10A so as to
move in the upper and lower directions. The lower end portion of
the outer mast 10A is mounted on the front side of a truck body 20
so as to fluctuate. Reference numeral 348 denotes the
above-mentioned tilt cylinder mounted to the front portion of truck
body 20. A piston 348P of the tilt cylinder 348 is joined to the
outer mast 10A so that the tilting angle in the forward and
backward directions of the upright 10 can be adjusted. Reference
numeral 346 denotes the above-mentioned lift cylinder mounted on
the central portion between the pair of uprights 10A, wherein the
piston 346P thereof is joined to the inner mast 10B can be adjusted
through a chain wheel supporter 10S so that the height of the inner
mast 10B in the upper and lower directions. Reference numeral 12
denotes a chain wheel rotatably mounted on the upper end of the
piston 346P. A chain 12C is fitted over the chain wheel 12. The one
end of the chain 12C is joined to the outer mast 10A or the lift
cylinder 346. The other end of the chain 12C is joined to a movable
member 16 slidably fitted into the inner mast 10B or a fork 18
supported by the movable member 16.
Reference numeral 18F denotes a top portion or free end of the fork
18. A load designated by reference numeral 40 is mounted on a
horizontal portion 18H of the fork 18. Reference numeral 24 denotes
a steering wheel for usual running control. Reference numeral 26
denotes a seat for an operator. Reference numerals 28F and 28B
denote a front wheel and a rear wheel, respectively.
Accordingly, when the lift cylinder 346 becomes operative, the
inner mast 10B elevates. According to this movement, the fork 18
which is attached to the chain 12C moves upwards along the inner
mast 10B. As a result, a load 40 mounted on the fork 18 is lifted.
FIG. 3 shows a detail of the portion with which the above-mentioned
lifting height sensor 102 is associated. The lifting height sensor
102 comprises a disk 102S having a plurality of slits coaxially
mounted to the chain wheel 12C and a sensor unit 102D, which may be
an electromagnetic type, in the embodiment, for instance,
consisting of a light source and a light detector (not shown). The
slitted disk 102S rotates in accordance with the rotation of the
chain wheel 12. The number of the slits passing the light source
end detector is detected by the sensor unit 102D. More
particularly, the sensor unit 102D produces a pulse signal
corresponding to the number of the slits, thereby detecting the
lifting height.
FIG. 4 is a block diagram, simplified for purposes of explanation,
wherein the same reference numerals shown in FIG. 1 denote
corresponding constituent members. Referring to FIG. 4, reference
numeral 232S denotes a push-button switch for starting an automatic
lifting height. When the push-button switch 232S is switched on,
the lifting height control is effected due to lifting height data
stored in a microcomputer unit 230C comprising e.g. the
above-mentioned control command producing circuit 240. Reference
numeral 232'B referred to later denotes an address selection key
switch for selecting a desired shelf position when the piling and
unloading of a load is effected.
However, a loading and unloading operation is effected with a fork
lift truck in which the above-mentioned control device for loading
and unloading mechanism is incorporated.
In the prior art, the storing of lifting height data is effected in
the microcomputer 230 in such a manner that that independent
address is assigned in connection with the height positions Hh and
Hl showing the same height of the shelf labelled by A as shown in
FIGS. 5A and 5B. Accordingly, two addresses are required with
respect to the same shelf. For this reason, it is necessary for an
operator to memorize a large number of addresses. As a result, it
is likely that there will occur an error in the handling thereof.
Further, when the piling of the load 40 on the shelf of FIG. 5A is
required, the following drawback is pointed out.
In connection with a situation wherein the load 40 is lifted
upwards to the height of Hh and the case that the load 40 is lifted
downwards from the height Hh to the height Hl, it is necessary for
an operator to actuate the address selection key switch 232'B for
each of these occurrences because the address of Hh is different
from that of Hl prior to starting of the lifting height operation.
Accordingly, this operation is troublesome for an operator.
The first embodiment has solved this problem. The feature according
to the first embodiment of the invention resides in that the two
positions (e.g. Hl, Hh in FIGS. 5A and 5B) can be stored in the
same address of the memory in the microcomputer 230. For instance,
with reference to FIGS. 5A and 5B, at the height position of the
fork 18 shown in FIG. 5A, the position of Hl at which the load 40
is not mounted on the fork 18 is stored in an address A shelf
corresponding to a shelf A. Further, at the height position of the
fork 18 shown in FIG. 5B, the position Hh, at which the load 40 is
mounted on the fork 18 through the palet 42, is stored in the
address A of the h-memory corresponding to the shelf A.
The memory of these lifting height positions Hl and Hh is effected
in accordance with the flow chart shown in FIG. 6. For instance, in
connection with the lifting height memory of Hl shown in FIG. 5A,
at the step S.sub.1 the address corresponding to shelf is selected
as address A with the address selection key 232'B. When the A
address is selected, the judgement is made at step S.sub.2 as to
whether the load 40 is mounted on the fork 18. As a result, it is
judged that there exists no load. Accordingly, a lifting height
memory key (not shown) is pressed as shown in the step S.sub.3.
Thus, the lifting height of Hl is stored in the 1-memory. Further,
in connection with lifting height memory of Hh shown in FIG. 5B,
the address A corresponding to shelf A is selected at the step
S.sub.1. In this instance, it is judged that there exists load at
the step S.sub.2. Accordingly, a lifting height memory key is
pressed, and the lifting height of Hh is stored in the h-memory, as
shown in the step S.sub.4.
In this instance, the judgment as to whether there is a load on the
fork may be effected by the operator. Alternately, instead of the
judgement of the operator, a limit switch responsive to the
presence or absence of the load on the fork or a pressure switch
responsive to inner hydraulic pressure of the lift cylinder
supporting the cylinder may be used. As a result of the sensing
with these switches, if the load is present, h-memory is
automatically selected, while if the load is absent, l-memory is
automatically selected.
The flow chart for lifting height control is shown in FIG. 7. At
the step S.sub.1, the selection of the address is effected. At the
step S.sub.2, the judgement is made as to whether there exists a
load on the fork. If it is judged that there exists no load, the
procedure is shifted to the step S.sub.3. The lifting height
operation starts so that the lifting height reaches the lifting
height value Hl stored in the l-memory as shown in the step
S.sub.5. If it is judged that there exists a load, the control is
shifted to the step S.sub.4. The lifting height operation starts so
that the lifting height reaches the lifting height value Hh stored
in the h-memory as shown in the step S.sub.5. In this instance, as
the switch for starting the lifting height control, a two position
switch is used. When the switch is closed on the desired side
(H.sub.l or H.sub.h), the control is started to the stored height
positioned on the side of closing direction selected by the switch.
The function designated by the steps S.sub.1 to S.sub.4 can be
executed with the switch for starting the lifting height control.
Accordingly, instead of the switch 232S for starting automatic
lifting height shown in FIG. 4 the above-mentioned switch for
starting lifting height control is incorporated with the
microcomputer 22. The lifting height operation for storing lifting
height data and the subsequent automatic lifting height operation
required for loading and unloading work can be automatically
effected.
If the above mentioned lifting height data storage is completed,
the subsequent procedure will be effected by the following
steps:
(1) When it is required to carry a load and place it on the shelf A
shown in FIGS. 5A and 5B, in front of the shelf,
a. The address of a shelf is selected. (with the address selection
key provided on the consol panel or the key board of the
microcomputer)
b. When the switch for starting the lifting height control is
shifted to the switching position for storing Hh, the fork 18 moved
to the height Hh.
c. The fork 18 is advanced forward so that the load 40 can be
placed on the shelf A. (see FIG. 5B)
d. When the switch for starting the lifting height control is
shifted to the switching position for storing Hl, the fork moves to
the height Hl so that the load is placed on the shelf A. (see FIG.
5A)
(2) When it is required to pick up a load 40 from the shelf, in
front of the shelf,
a. The address of A shelf is selected (with the address selection
key switch provided on the consol panel or the key board 246 of the
microcomputer 230).
b. When the switch for starting the lifting height control is
shifted to the switching position for storing Hl, the fork 18 moves
to the height Hl.
c. The fork 18 is advanced so as to put it into the palet 42. (see
FIG. 5A)
d. When the switch for starting the lifting height control is
shifted to the switching position for storing Hh, the picking-up of
the load 40 is completed. (see FIG. 5B)
As is clear, the first embodiment of the invention is capable of
storing the two positions (for instance, Hl, Hh) in the same
address of the memory 244 of the microcomputer 230. Accordingly,
one shelf can correspond to one address. As a result, in comparison
with the prior art, it is easy for an operator to bear in mind the
relationship between the shelf and the address. It is sufficient
for an operator to bear in mind a small number of addresses,
thereby making it possible to eliminate erroneous actuation of the
address selection key switch. Further, the embodiment of the
invention makes it unnecessary to actuate the address selection key
switch for each operation as in the prior art. As a result, there
is no erroneous operation, thereby enabling an increase in the
working efficiency.
Reference is made to the second embodiment of the invention.
FIG. 8 is a block diagram showing an automatic loading and
unloading mechanism constituted wherein the same reference numerals
used in FIG. 1 denote corresponding parts, respectively. Reference
numeral 106 denotes a hydraulic pressure sensor for sensing the
hydraulic pressure of the lift cylinder 346. Since the hydraulic
pressure of the lift cylinder 346 elevates in the loaded condition,
the pressure sensor 106 feeds an output "1" to the microcomputer
230. On the contrary, in the unloaded condition, the pressure
sensor 106 feeds an output "0" to the microcomputer 230. The pulse
output of the lifting height sensor 102 is also fed to the
microcomputer 230. In the microcomputer 230, the pulse input is
counted by the lifting height counter 222 (see FIG. 1) provided in
the interface circuit 220. At the same time, a predetermined
arithmetic processing is effected based on the output of the
lifting height counter 222 in the microcomputer 230 to calculate
the lifting height distance. The calculated height distance is
displaced on the key board 246.
In the above mentioned automatic loading and unloading mechanism,
when the operator calls the specified address to which the lifting
height data is stored by the actuation of the key board, and then
pushes the push-button switch for starting automatic lifting height
control, the microcomputer 230 produces a control command for
hydraulically driving the control valve 342 through the driving
motors 322M on the basis of the information obtained by the
pressure sensor 106, the lifting height sensor 102 to effect an
automatic lifting height control by operating the cylinder 346 so
that the lifting height is equal to the data previously stored.
Assuming that, when a specified address is called in order to
effect the automatic lifting height control, the operator
erroneously calls an address of the memory in which the lifting
height data is not stored, there occurs the following problem:
Since the concerned memory erroneously called does not contain the
data required for regular automatic lifting height control, if the
push-button switch for starting automatic lifting height control is
pushed, the lifting height operation of the fork 18 is effected
based on the data having no relation with the lifting height
control, with the result that there occurs a dangerous
accident.
In the present embodiment, when the lifting height data is stored
in the address the following technique is employed. Assuming that a
microcomputer has a memory area in which 16 bits of data can be
stored. As shown in FIG. 9, the numeral, for instance coded signal
1010, showing that the lifting height data is stored in advance in
a memory area II, designated as a vacant area, in which 4 bits of
data can be stored according to need, the area excluding a memory
area I comprised of 12 bits used for storing the lifting height
data. When effecting an automatic lifting height control, prior to
the actuation of the push-button switch for starting an automatic
lifting height control, the operator determines whether the numeral
indicating that the lifting height data is stored in the specified
address. The operator pushes the button for starting an automatic
lifting control solely upon determining the presence of the
numeral.
FIG. 10 is a flow chart for effecting these procedures by program
control. The method comprises the steps of designating the address
(step S.sub.1), examining or judging as to whether there exists a
numeral showing that the lifting height data is stored in the
address (step S.sub.2), if the numeral is not found, producing an
alarm output (step S.sub.3), and returning the program execution to
the main loop (step S.sub.4). In this instance, since the lifting
height data is not stored, the storage of the data necessary for
lifting height control is required. On the other hand, if the
numeral is found at the step S.sub.2, as shown in the step S.sub.5,
the concerned stored lifting height data is read out to effect an
automatic lifting control (step S.sub.6), and then the program
execution is returned to the main loop shown in the step S.sub.4 to
execute a program in connection with the other control.
Accordingly, when the numeral showing the memory cannot be found,
the automatic lifting height control cannot be effected, even if
the push-button switch for starting the automatic lifting height
control is pushed. As a result, there is little possibility that
the fork moves to the position erroneously designated, thereby
improving operating safety. The operator recognizes by the alarm
signal that the lifting height data is not stored in the
address.
In the second embodiment, a certain number (e.g., a binary coded
numeral) indicating the data, is stored in the vacant area II of
the memory corresponding to the same address as that of the memory
area I in which the lifting height data is stored. In addition to
such a storing method, the numeral indicating data is stored in the
vacant bit of an address relevant to the address in which the
lifting height data is stored. In this instance, it is sufficient
to check as to whether the numeral indicating memory is present in
the relevant address when an automatic lifting height control is
effected.
Reference is made to the third embodiment of the invention.
It is necessary to move the fork 18 to the predetermined height
when the lifting height data is stored in memory 244 of the
microcomputer 230 with the key board 246. In this instance, if the
fork 18 is lifted to the maximum height, the hydraulic pressure of
the lift cylinder 346 increases even in the unloaded condition. As
a result, the hydraulic pressure sensor 106 is turned on. The
microcomputer 230 erroneously recognizes that it is a loaded
condition. For this reason, even if an operator causes to store the
lifting height data in the unloaded condition into the
microcomputer 230 by the actuation of the key board 246, the data
is automatically stored in the address allotted to the loaded
condition. As a result, there occur inconveniences or serious
errors in the automatic lifting control either in the unloaded or
loaded conditions. Further, assume that the lifting height data is
stored in the microcomputer 230 under the condition that the fork
18 is lowered to ground. When the thickness of the horizonal
portion 18H of the fork 18 is large as compared with a conventional
fork, even if attempting to lower the fork 18 to a stored position
corresponding to ground by effecting an automatic lifting height
control, it is actually impossible to lower the fork 18 to the
stored ground position. For this reason, there is a drawback that
the command indicative of lowering of the fork 18 is continuously
fed from the microcomputer 230, thereby resulting in a situation in
which the system is unable to shift to the subsequent
operation.
The first embodiment of the present invention has solved these
problems, which will be explained with reference to FIG. 8. The
upper limit and the lower limit to be stored are setted in the
microcomputer 230 as shown by labels X.sub.H and X.sub.L in FIG.
11. In this instance, the upper limit to be stored is selected so
that it is slightly lower than the lifting height corresponding to
the output of the load sensor 106 associated with the lift cylinder
346 in the unloaded condition, while the lower limit to be stored
is selected so that it is slightly larger than that of maximum
value of the thickness of the horizontal portion 18H of the fork
18. The microcomputer 230 executes a program based on a flow chart
shown in FIG. 12. At the tep S.sub.1, the concerned memory routine
is looked up in the main loop for controlling various kinds of
controls required for, such operations as the lifting height
control of the fork 18, stored in ROM 244B of the microcomputer
230. If the memory routine is found by looking-up, at the step of
S.sub.2, the concerned memory routine is called. At the step
S.sub.3, the comparison between the stored lower limit of the
lifting height value and the present lifting data obtained from the
lifting height sensor 102 is effected with the memory routine. At
the step S.sub.4, if the result is minus, that is, the present
lifting height value is above the stored upper limit of the lifting
height, the execution of the program is returned to the main loop
at the step S.sub.1, for a second time. On the contrary, if the
result is equal to zero or plus, that is, the present lifting
height value is lower than the stored upper limit of the lifting
height, the execution of the program is shifted to the step
S.sub.5. At the step S.sub.5, the comparison between the lower
limit of memory previously stored and the present lifting height
value is further effected. At the step S.sub.6, if the result is
equal to zero or is positive, that is, the present lifting height
value is lower than the lower limit of the memory or equal thereto,
the program execution is returned to the main loop at the step
S.sub.1, for a second time. On the contrary, if the result is
negative, the present lifting height value is higher than the lower
limit of the memory, the program execution is shifted to the step
S.sub.7. At the step S.sub.7, the signal "memory OK" showing that
it is possible to store the lifting height data is transferred to
the memory subroutine. Thus, it is possible to store the
predetermined lifting height value in the microcomputer 230.
FIG. 13 is a block diagram for effecting the above mentioned
control based on the program shown in FIG. 12. As stated above, the
lifting height counter 222 is provided at the interface 220 shown
in FIG. 1. In the embodiment, the lifting height counter 222
comprises three up-down counters 222A, 222B and 222C. The first
counter 222A counts pulse output fed from the lifting height sensor
102. The CPU 242 effects a calculation based on the counted value
to produce a signal indicative of lifting height. The corresponding
lifting height data is displayed on the key board 246. In order to
preset the above-mentioned upper and lower limits, there are
provided the second counter 222B for presetting the upper limit of
the lifting height, for instance, 2.8 m and the third counter 222C
for presetting the lower limit, for instance, 8 cm.
A reset switch 222R is switched on under the condition that the
fork 18 is placed on ground. Thereby, the first counter 222A is
cleared and the upper and lower limits of lifting height are set to
the second and third counters 222B and 222C. Then, the lifting
height operation of the fork 18 is effected to move the fork 18 in
the upward and downward directions. According to this operation,
the first counter 222A effects up-count at the time of elevation of
the fork 18 to feed up-signal labelled by Su each subtracting input
terminal I.sub.R of the second and third counters 222B and 222C.
Thus, the reduction is effected in the second and third counters
222B and 222C. Likewise, at the time of lowering of the fork 18,
the first counter 222A effects down-count to deliver to each adding
input terminal I.sub.A of the down-signal labelled by S.sub.D to
the second and third counters 222B and 222C. Thus, addition is
effected in the second and third counters 222B and 222C.
Accordingly, when the lifting height value of the fork 18 is above
the stored upper limit, the output of the second counter 222B is
negative to produce a logical output "1". On the contrary, when the
lifting height value of the fork 18 is higher than the stored lower
limit, the third counter 222C is negative to produce a logical
output "1". The output of the third counter 222C is inverted by the
NOT gate 224. On the other hand, when the fork 18 reaches the
position equal to the stored lower limit or lower than that, the
output of the third counter 222C is "0". The output of the third
counter 222C is inverted by the NOT gate 224. As a result, the
logical signal "1" is fed to the OR gate 226. Thus, when the fork
18 is above the stored upper limit, equal to or below the stored
lower limit, either of the input of the OR gate 226 is "1". As a
result, the OR gate 226 produces a memory inhibiting signal, even
if the operator attempts to set a memory of lifting height to the
microcomputer 230 with the key board 246, thereby making it
impossible to store a lifting height data.
According to the third embodiment of the present invention, when
the position of the fork 18 is above the upper limit previously
stored in the microcomputer 230, or below the lower limit stored
therein, that is, the fork 18 is not within the range of permitted
lifting height, the memory setting of the lifting height data is
inhibited. Accordingly, the value stored in the microcomputer 230
by memory-setting of the lifting height data in the unloaded
condition is erroneously identified with the value stored in the
loaded condition. Even if the automatic lifting height control is
effected with a fork lift truck having a fork of which thickness is
large, there does not occur the situation wherein the fork 18
cannot be lowered to the lifting height previously set, thereby
making it possible to smoothly effect the automatic lifting height
control.
Reference is made to the fourth embodiment of the present
invention. The second embodiment has solved the problem occuring
when a lifting height speed control is effected by controlling a
servo driving system for actuating a lift cylinder. For better
understanding of the second embodiment, the method of controlling
the lifting height speed will be described with reference to FIG.
14. Reference numeral 322 denotes the above-mentioned first
actuator which becomes operative in accordance with a command
signal S.sub.1 indicative of opening angle fed from the
microcomputer 230. As stated above, the actuator 322 comprises a
driving motor 322M, transistors 322T.sub.1 to 322T.sub.4, and
clutch 322C. The valve opening angle of the first control valve 342
is controlled by correction signals S.sub.2 and S.sub.3 fed from
the actuator 322. The lift cylinder 346 is controlled by an output
signal S.sub.4 fed from the first control valve 342. Thereby, the
piston 346P becomes operative to effect a lifting height control.
Reference numerals 345T and 345P denote hydraulic oil tank and
hydraulic pump, respectively. Reference numeral 345D denotes a
driving circuit for the hydraulic pump 345P. The driving circuit
345D comprises, for example, an engine or a motor. According to the
device thus constructed, (mainly, within the region of medium and
low speeds) the follow-up control of the lifting speed (or lowering
speed) to the predetermined value is effected by adjusting the
opening angle of the first control valve 342 through the driving
motor 322M and clutch 322C. The setting speed is stored in the
microcomputer 230 with the above-mentioned data setting means 246
e.g. key board. Tne stored setting speed is compared with the
actual speed S.sub.5f being fed from the lifting height sensor 102.
The command signal S.sub.1 indicative of valve opening angle
corresponding to the deviation based on the comparison, is fed to
the actuator 322 to control the driving motor 322M. The opening
angle of the first control valve 342 is corrected by the correction
signals S.sub.2 and S.sub.3 fed from the actuator 322. The lift
cylinder 346 is actuated by the control signal S.sub.4 to effect a
lifting height speed control.
With the above-mentioned arrangement, there exists a response
delay. After a correction signal for increasing speed is produced,
it takes 10 milliseconds or 100 milliseconds until the driving
motor 322M rotates to open the valve with the result that the
lifting speed actually increases. Another drawback is pointed out
as follows: The valve opening angle command for increasing the
speed is continuously fed to the driving motor 322M until the
actual lifting speed reaches the setting value newly set for
increasing a speed. Particularly, in this instance, in the region
where the valve opening angle is small, the change of the speed
with respect to the valve opening angle command is abrupt.
Accordingly, the speed of the driving motor 322M abruptly increases
to increasingly open the first control valve 342, with the result
that the lift cylinder 346 is elevated quickly. When the actual
lifting height speed reaches the setting value, the deviation is
equal to zero. At the same time, when the command for stopping the
driving motor 322M is fed to the actuator 322, the driving motor
322M is stopped under the condition that inertia is applied
thereto. Accordingly, the valve opening angle at that time is
larger than that corresponding to the setting lifting height speed
by an amount due to the inertia. As a result, the actual lifting
height speed is too fast as compared with the setting lifting
speed. Accordingly, the equilibrium between the speed sensed by the
lifting height sensor 102 and the setting speed is broken. As a
result, a valve opening angle command S.sub.1 having a negative
polarity is produced by the microcomputer 230 due to the deviation.
An inverse operation occurs in the direction of closing the first
control valve 342. From the time when the command for stopping the
driving motor is produced, the speed of the lift cylinder 346
gradually attenuates varying or vibrating in the positive and
negative directions under the condition that the changed lifting
height speed serves as a boundary, and then reaches the
predetermined lifting height speed after the predetermined time
passes.
As stated above, the drawbacks of the prior art lifting height
speed control are pointed out as follows: In addition to the
response delay, there is a lack of smoothness and stability when
effecting a speed control due to the vibration of the lifting
height speed when the setting value is altered.
The fourth embodiment has solved these problems, which will be
described with reference to FIG. 15. In FIG. 15, the same reference
numerals denote corresponding parts, respectively, for which an
explanation is omitted.
In the automatic speed control system, a major loop for lifting
height speed control is labelled by L.sub.1 and a minor loop for
valve opening angle is labelled by L.sub.2. Reference numeral 262A
denotes a digital to analog converter (D-A converter) for
converting a digital command signal S.sub.6 fed from the computer
230 to an analog signal S.sub.7 indicative of the valve opening
angle setting signal. Reference numeral 262B denotes a comparing
circuit for comparing the setting signal S.sub.7 with a sensed
voltage of the servomotor driving circuit referred to soon.
Reference numeral 262C denotes an amplifier for amplifying the
difference output signal S.sub.8 fed from the comparing circuit
262B. The driving motor 322M becomes operative in accordance with
the amplifying signal S.sub.9 fed from the amplifier 262C.
Reference numeral 322P denotes a potentiometer cooperative with the
driving motor 322M. The feed back signal S.sub.10 fed from the
potentiometer 322P is fed to the comparing circuit 262B. Reference
numeral 342W denotes a toothed wheel which becomes operative in
cooperation with the clutch 322C. Reference numeral 342L denotes a
lever fixed to the axle of the toothed wheel 342W. The lever 342L
is mounted to the one end of each of springs 342S.sub.1 and
342S.sub.2. The other ends of these springs 342S.sub.1 and
342S.sub.2 are fixed to a stationary member (not shown). A spool
(not shown) for opening and closing the valve, which communicates
with the duct 342C is disposed within a valve unit 342V. The spool
is joined to the lever 342L.
With the above mentioned lifting height control device, the digital
command signal S.sub.6 fed from the microcomputer 230 is converted
into an analog signal by the D/A converter 262A. The analog signal
serving as a valve opening setting signal, S.sub.7, is fed to the
comparing circuit 262B. The servomotor driving circuit 322' becomes
operative in accordance with the amplified signal S.sub.9 due to
the deviation between the valve opening angle setting signal
S.sub.7 and the feed back signal S.sub.10. Thus, the predetermined
rotational angle of the driving motor 322M is determined. That is,
when in accordance with the amplified signal S.sub.9 corresponding
to the valve opening setting signal S.sub.6, the transistors
322T.sub.1 and 322T.sub.2 become operative, the driving motor 322M
rotates in the forward direction. Conversely, when the transistors
322T.sub.3 and 322T.sub.4 becomes operative, the driving motor 322M
rotates in the backward direction. According to the rotational
angle of the driving motor 322M, the lever 342L is rotated through
the clutch 322C and the toothed wheel 342W. Thus, the valve opening
angle is determined. As a result, the moving speed of the piston
346P of the lifting cylinder 346 is determined. According to the
moving speed of the piston 346P, the pulse signal S.sub.5f fed from
the lifting height sensor 102 constituted as a pulse generator is
fed to the microcomputer 230.
The predetermined setting speed signal is set in the memory 244 of
the microcomputer 230. The microcomputer 230 effects a comparing
calculation between the actual speed of the piston 346P and the
setting speed to output the digital command signal S.sub.6. The D/A
converter 262A produces a voltage proportional to the command
signal S.sub.6 to feed it to the comparing circuit 262B. In the
comparing circuit 262B, the comparison between the voltage
(S.sub.7) and the feed back signal S.sub.10 is effected. The
control of the valve opening angle is effected under the condition
that the output of the comparing circuit 262B serves as a control
command of the minor loop. Thus, the lifting height control is
effected in accordance with the above-mentioned operation.
The speed of the fork 18 is shown as curves l.sub.1 and l.sub.2 in
FIG. 17 where Symbol l.sub.1 denotes a characteristic curve in the
unloaded condition, and l.sub.2 a characteristic curve in the
loaded condition. As understood from FIG. 17, the fork 18 is not
elevated at the opening angle of .theta..sub.0 even in the unloaded
condition. At the angle of .theta..sub.1, the lifting speed is
placed in full speed condition in the unloaded condition, while in
the loaded condition, the fork 18 does not move at all. At the
angle of .theta.max. which is maximum opening degree, the lifting
speed thereof is placed in full speed condition in the loaded
condition. For this reason, in the present embodiment, it is
designed so that the angle ranging from .theta..sub.0 to
.theta.max. is divided into a plurality of steps, for instance, 50
steps, to output a command signal corresponding to the opening
angle of the valve from the microcomputer 230.
FIG. 16 is a flowchart showing an execution of the program of the
microcomputer 230.
When the signal S.sub.5f indicative of the sensed speed is fed back
to the microcomputer 230, at the step S.sub.1, it is determined
whether the timer setting time is passed or not. If the timer
setting time has not passed, the program execution is returned to
the step S.sub.1 for a second time.
If the predetermined time interval, e.g. 20-30 millisecond set in
the timer is passed, the comparison between the present speed and
the reference speed is effected at the step S.sub.2. If the present
speed is not larger than the reference speed, the execution is
shifted to the step S.sub.3 to deliver a command for increasing the
speed by one step. When the present speed is larger than the
reference speed, the program execution is shifted to the step
S.sub.4 to produce a command for decreasing the speed by one step.
When the present speed is equal to the reference speed, the command
for maintaining the present condition is produced at the step
S.sub.5. Thus, when the program execution at the step S.sub.3,
S.sub.4 and S.sub.5 is completed, the timer resetting operation is
effected at the step S.sub.6. Thereafter, the timer starting
operation is effected at the step S.sub.7. The program execution is
returned to the step S.sub.1. The same procedure will be
repeated.
The program execution for comparing the setting value and the
present speed in the microcomputer 230 is stated above. Turning now
to FIG. 15, the operation of the lifting height speed control
device according to the present embodiment is described, assuming
that the correction of the lifting height speed is effected under
the condition that the fork 18 is controlled at the predetermined
lifting height speed.
When the speed sensing signal S.sub.5f corresponding to the moving
speed of the piston 346P obtained by the lifting height sensor 102
is fed to the microcomputer 230, the judgement as to whether the
predetermined time set by the timer passes or not is effected in
accordance with the flowchart shown in FIG. 16. Thereafter, the
comparison between the setting speed and the present speed is
effected. If the present speed is less than the setting speed, as
shown in FIG. 16 the microcomputer 230 produces the binary coded
command signal S.sub.6 for increasing the speed by plus one step.
If the present speed is above the setting signal, the microcomputer
230 produces the coded command signal S.sub.6 for decreasing the
speed by minus one step. If the present speed is equal to the
setting signal, the microcomputer 230 produces the coded command
signal S.sub.6 for maintaining the speed. In the D/A converter
262A, the command signal S.sub.6, which is a coded signal, as for
example, 0 to 50 in FIG. 18 is analog-converted to produce a
voltage signal corresponding thereto. This voltage signal serves as
a valve opening angle setting signal S.sub.7. As stated above, the
valve opening setting signal S.sub.7 is rendered to the minor loop
L.sub.2 as the control command. Thus, the first control valve 342
is controlled. According to this control, the lifting height speed
is controlled.
According to the second embodiment of the invention, the subsequent
correction signal is not increased or decreased solely by one step
due to the difference between the actual speed and the setting
speed, in a time delay of about 10 milliseconds set by the timer
after the preceeding correction signal is produced. Accordingly,
after the correction signal is produced and after there then occurs
a change of the speed due to the correction, the subsequent
correction is effected. As a result, an excessive correction can be
eliminated. Further, since the adjusting step of the valve opening
angle is sufficiently small, the rotational angle of the driving
motor 324M is small with respect to each correcting operation. As a
result, in the stopping operation of the driving motor 322M,
effected due to a stopping command which is produced when the speed
reaches the setting value, there is little possibility of producing
an excessive rotation of the driving motor 322M due to inertia.
Further, the changing step of the valve opening angle is
sufficiently small, thereby making it possible to prevent the speed
from being abruptly changed. Accordingly this brings about a
stabilized lifting height control.
Reference is made to the fifth embodiment of the invention. In this
embodiment, the lifting height speed control device shown in FIG.
15 is employed. The same reference numerals used in FIG. 1 denote
corresponding parts, which explanation will be omitted.
A program for an automatic lifting height control is stored in the
microcomputer 230. When a push button for starting lifting height
operation is pushed, the microcomputer 230 feeds a control signal
to the first control circuit 262 (see FIG. 1) in accordance with
the program for lifting height control. The control circuit 262
feeds a control command indicative of valve opening angle to the
base of each of transistors 322T.sub.1 to 322T.sub.4 constituting a
servomotor driving circuit 322 to effect an ON-OFF control of these
transistors. Thus, the driving motor 322M is controlled, so that
the first control valve 342 is actuated similarly to the
above-mentioned embodiment.
As a result, the lift cylinder 346 lifts or lowers the fork in
accordance with the upward and downward movement of the piston 346P
of the lift cylinder 346.
The microcomputer 230 senses the lifting height and the speed of
the fork 18 due to the pulse output fed from the lifting height
sensor 102. On the basis of these sensed data, the microcomputer
230 executes a program for effecting an automatic lifting height
control.
However, in such an automatic lifting height control device to
which microcomputer 230 is applied, if the fork 18 is attempted to
be suddenly stopped in the condition of the high speed when the
height of the fork 18 is varied from one height to the other height
and then is stopped thereat, it is likely that the load 40 mounted
on the fork 18 will become misshapen. Therefore, it is desirable to
slowly decelerate the fork 18. When the height of the fork 18 is
changed, there occurs a situation in which it is necessary to lower
the speed at the time of attitude of the load 40 which is easily
misshapen. In such a case, it is necessary to effect a follow-up
control of the speed. There is a time delay until the lifting speed
follows up to the setting value by the speed control command fed to
the first control circuit 262 from the microcomputer 230. Further,
the actual speed is calculated by the frequency of the pulse
output, which is provided by the lifting height sensor 102,
occuring every time the fork 18 moves for a predetermined interval.
However, it takes much time to sense the lifting height speed. For
this reason, there is an inconvenience in an automatic speed
control immediately before the objective height.
The fifth embodiment of the invention has solved these problems,
which will be described with reference to the flow chart of FIG. 19
illustrating an embodiment of an automatic speed control
immediately before objective height. At the step S.sub.1, the
difference between the setting objective height Hs and the present
height Hc is calculated. At the step S.sub.2, it is judged whether
the lifting height reaches the setting objective height Hs. As a
result, if the present lifting height reaches the setting objective
height Hs, the automatic speed control is completed. On the
contrary, if the present lifting height does not reach the setting
objective height Hs, the program execution is shifted to the step
S.sub.3. The data pattern relating the setting speed SPs with the
absolute value .vertline.H.vertline. of the difference between the
setting objective height Hs and the present height Hc is stored in
the microcomputer 230. An example of the data pattern is shown by
(A) and (B). At the step S.sub.3, a reading operation of the
setting speed SPs with respect to the absolute value
.vertline.H.vertline. is effected. Then, at the step S.sub.4, the
read operation of the present speed SPc is effected. The present
lifting height is calculated by counting the pulses generated every
time the fork moves a predetermined distance, generated by the
lifting height sensor 102. On the other hand, the present speed is
calculated by measuring an interval of pulse duration. The measured
time is as shown in FIG. 20A from the rising of the pulse train (or
the falling thereof) to the subsequent rising of the pulse train
(or the falling thereof). The timer pulse train as shown in FIG.
20B is obtained by a software timer.
The procedure for obtaining the timer pulse train will be described
with reference to FIG. 20C. First of all, at the step S.sub.1, the
judgement as to whether the status of the sensor pulse train is "1"
is effected. At the step S.sub.2, the waiting operation, for a
predetermined time interval, e.g. 1 m sec is effected. At the step
S.sub.3, the timer count value is advanced by one. At the step
S.sub.4, the judgement as to whether the status of the pulse train
is "1" at that time is effected for a second time. Until the status
of the pulse train is "1", the program shown by steps S.sub.2 and
S.sub.3 continues to be executed. When the status of the pulse
train is "1", as shown in step S.sub.5, the value of the timer
count is calculated. Thus, measuring time information is
obtained.
The processing at the step S.sub.4 shown in FIG. 19 is stated
above. The remaining processing for a program executed in
accordance with the flow chart will be described as follows:
At the step S.sub.5, (SPc-SPs)/SPs (%) is calculated. The program
execution is branched as shown in step S.sub.7 proportional to the
difference due to the branching command as shown in the step
S.sub.6. When the difference is small (for instance, within 10%),
the maintaining present speed command is produced as shown in the
step S.sub.71. When the difference is from +10% to +20%, the
command (for decreasing the valve opening angle by one step with
respect to the present valve opening angle) for decreasing the
speed by one step is produced as showin in the step S.sub.72. One
step is defined as one interval obtained by equally dividing the
predetermined region of lift valve opening angle into multiple
steps, as shown in FIG. 18. When the difference is above 20%, the
command for decreasing speed by two steps with respect to the
present speed command is produced as shown in the step S.sub.73.
When the difference is from -10% to -20% or above -20%, the command
for increasing the speed by one step or the command for increasing
the speed by two steps is produced as shown in steps S.sub.74 and
S.sub.75, respectively. The program shown by the flow chart shown
in FIG. 19 is executed by the microcomputer 230. The speed command
signals corresponding to the steps S.sub.71 to S.sub.75 are fed to
the first control circuit 262 shown in FIG. 15 by the microcomputer
230. After a constant retarded time as shown in the step S.sub.8,
the program execution shown in FIG. 19 is repeated. When the
constant speed control is effected, SPs shown in FIG. 19 is a
constant value. The speed control command as shown in FIG. 15 is
produced according to the magnitude of the actual speed SPc.
As stated above, when the flow chart shown in FIG. 19 is executed,
the present speed is obtained by a software timer as shown in FIGS.
20A, 20B and 20C instead of frequency of the sensor output.
Accordingly, it is possible to promptly sense the present speed.
For this reason, the follow-up control in the automatic speed
control system immediately before the objective height is effected
promptly because of the fact that the sensing of the lifting height
speed is quicked.
Reference is made to the sixth embodiment of the invention.
In the above mentioned fork lift truck, as shown in FIG. 2, during
automatic lifting height control, when the fork 18 does not reach
the objective height (object position), there occurs a situation in
which the control is interrupted and stopped by the judgment of an
operator. In the prior art, such a stopping actuation is effected
with the operation of an emergency stop button or a manual
lever.
However, when the actuation is effected with the emergency stop
button or the manual lever, the shift operation of a spool provided
in the first control valve 342 to the neutral position is abruptly
effected. For this reason, lifting or lowering speed suddenly
becomes zero or suddenly varies. As a result, there occurs an
undesirable feeling. Alternately, the load may drop, resulting in a
serious accident.
The present embodiment has solved these problems, which is
explained with reference to accompanying drawings. In the present
embodiment, the automatic lifting height control device used in the
third embodiment is employed. FIG. 21 is a flow chart showing a
main program for an automatic lifting height control.
At the step S.sub.1, the absolute value .vertline.H.vertline. of
the difference between the objective height (Hs) at which the top
portion of the fork 18 arrives and the present height (Hc) is
detected. At the step S.sub.2, the judgement is made as to whether
the absolute value .vertline.H.vertline. is equal to zero or not.
If the absolute value .vertline.H.vertline. is equal to zero, it is
judged that the fork 18 has reached the objective height.
Accordingly, as shown in the step S.sub.3, the command for stopping
the driving motor is produced. When the absolute value
.vertline.H.vertline. is not equal to zero, at the step S.sub.4, a
judgement is effected as to whether the absolute value is equal to
or less than 50 cm. If .vertline.H.vertline.>50 cm, at the step
S.sub.5, the command for maintaining the present speed is produced.
At the step S.sub.4, if the absolute value .vertline.H.vertline. is
equal to or less than 50 cm, the program execution is shifted to
the step S.sub.6. At the step S.sub.6, the judgement as to whether
the absolute value .vertline.H.vertline. is equal to or less than
20 cm is effected. If 50 cm>.vertline.H.vertline.>20 cm, at
the step S.sub.7, the medium speed control command output is fed to
the first control circuit 262. If .vertline.H.vertline..ltoreq.20
cm, at the step S.sub.8 low speed, as for example, very slow
control command output is fed to the first control circuit 262.
Thus, the first control circuit 262 delivers the servo valve
opening angle command signal corresponding to each input signal to
the transistors 322T.sub.1 to 322T.sub.4 constituting the
servomotor driving circuit 322' to control the driving motor 322M.
Thus, as understood from the description stated above, the first
control valve 342 and the lift cylinder 346 are controlled in
accordance with the output of the servomotor driving circuit 322'.
During such an automatic lifting height control, when the fork 18
does not reach the objective height, there occurs a situation in
which it is required to interrupt and stop the movement of the fork
18 according to the operator's will. In such a case, it is
desirable to slowly stop the fork 18.
The operation for slowly stopping the fork will be effected as
follows: (control mode)
(1) a judgement as to whether the control is effected at the high
speed, medium speed, or low or very slow speed is effected.
(2) If the control is effected at the high speed, the command for
the high speed is changed to the command for the medium speed.
(3) If the control is effected at the medium speed, the command for
the medium speed is changed to the command for the very slow
speed.
(4) If the control is effected at the very slow speed, the command
for stopping the movement is produced or the command for continuing
to effect the automatic control is produced.
(When the very slow control is effected immediately before the
objective height during an automatic control is effected, the
automatic control is continued.)
(5) When the control is entered into the control for slowly
stopping the driving motor,
(a) the driving motor is decelerated and stopped in a predetermined
retarded time on the basis of the following pattern; high
speed.fwdarw.medium speed.fwdarw.very slow speed.fwdarw.stop (the
method of changing the mode according to time).
(b) The control is effected depending on driving condition. For
instance, when the control is effected at the high speed, the
objective distance (objective position) Hs is altered to the
distance obtained by adding 50 cm to the present position and the
command is changed so that the medium speed control is effected. If
the the fork is within 20 cm with respect to the setting objective
position, the command is changed so that the very slow control is
effected and stopped at the objective position.
On the other hand, if the control is effected at the medium speed,
the setting is effected so that the objective position Hs is 20 cm.
Thus, the command is changed so that very slow speed is effected
and stopped at the object position. (The method of changing the
control mode according to the distance).
Reference is made to the methods as defined in the items (1) to (4)
and 5(b) with reference to FIG. 22.
A subroutine for slow stop interrupt command, that is, the method
defined in items (1) to (4) and 5(b) shown in FIG. 22 is set to the
main program stored in the microcomputer 230, which is shown in
FIG. 21. The microcomputer 230 is provided with a push-button
switch 232B for slow stop interrupt command. When the push-button
switch 232B is pushed, the slow stop interrupt command shown in
FIG. 22 is produced. The microcomputer 230 judges as to whether the
present speed is high, medium or low (very slow) at the step
S.sub.1 on the basis of the output of the lifting height sensor
102. If the speed is high, the program execution is branched to the
step S.sub.2. At the step S.sub.2, 50 cm is entered into the
objective height (Hs) and the program execution is shifted to the
main program for automatic lifting height. If the speed is medium,
the program execution is branched to the step S.sub.3. At the step
S.sub.3, 20 cm is entered into the objective height Hs and the
program execution is shifted to the main program shown in FIG. 21.
If the speed is very slow, the program execution is branched to the
step S.sub.4. As shown in the step S.sub.4, the main program for
automatic lifting height shown in FIG. 21 is continued under the
condition that the objective height Hs is the same as that of the
previous one. Thus, the microcomputer 230 executes the main program
for automatic lifting height shown in FIG. 21 on the basis of the
slow stop interrupt command shown in FIG. 22. The corresponding
control command signal is fed to the first control circuit 262 from
the microcomputer 230. Assuming that the fork 18 is lowering, the
top portion 18F of the fork 18 is completely stopped as shown in
FIG. 23A. Assuming that the fork 18 is lifting, the fork 18 is
stopped as shown in FIG. 23B.
During automatic lifting height control, when the push button
switch 232B for slow stop interrupt command provided in the
microcomputer 230 is switched on, the microcomputer 230 determines
the distance required for the stop of the fork 18 due to the speed
immediately before that time. The decelerating operation is
effected by gradually lowering the setting speed until the fork 18
reaches the object height. Thus, the fork 18 is completely stopped.
That is, the speed control is softly effected until the fork 18 is
placed in the stopped mode. Accordingly, this makes it possible to
eliminate a shock which may be caused when the fork 18 is stopped.
As a result, dropping of the load 40 does not occur.
According to the present embodiment, the slow stopping operation is
effected with the method defined in the items (1) to (4) and 5(b).
However, the present invention is not limited to this procedure.
This slow stopping operation can be performed with the method
defined in the items (1) to (4) and 5(a). In this instance, instead
of setting and judging due to the distance (steps S.sub.1 to
S.sub.4 and step S.sub.6 shown in FIG. 21, the steps S.sub.2 and
S.sub.3 shown in FIG. 22), it is sufficient to use the setting and
judging due to time. For instance, the lifting operation of the
fork 18 is exemplified. The following procedure is applicable to
the lowering of the fork 18. As shown in FIGS. 23A and 23B, due to
the actuation of the push-button switch 232B, the microcomputer 230
produces a command for decreasing the speed immediately before that
time by one step. The microcomputer produces a command for further
decreasing the step by one step in a predetermined time. Thus, the
fork 18 is completely stopped. Since the control for stopping the
fork is softly effected, the shock occuring when the fork is
stopped can be eliminated. As a result, the load 40 will not fall
down.
As is clear from the foregoing description, the control device
according to the present embodiment has the following
advantages:
During an automatic lifting height control, when slowly stopping
operation is required, the push-button switch 232B for slow stop
interrupt command is pushed. Thereby, the control for stopping
operation is effected by making good use of the method of
decreasing the lifting speed immediately before the push button
switch 232B is switched on as a function of time (method as shown
in the item 5(a)) or the method for decreasing the same as a
function of distance (method as shown in the item 5(b)) set in the
microcomputer 230. The suitable setting of the time and distance at
the time of utilizing the above-mentioned methods makes it possible
to prevent dropping of the load, thereby enabling smooth
stopping.
In FIGS. 22 and 21 embodiments in which the method featured by the
item 5(b) is employed, 50cm and 20 cm are used as the setting
distance. However, the distance is not limited to this value.
According to the situation of a load 40 placed on the horizontal
portion 18H of the fork 18, the above selected distance of 50 cm
and 20 cm can be suitably changed. On the basis of the modified
value, the microcomputer 230 freely adjusts the decelerating
speed.
Reference is made to the fifth embodiment of the present invention.
The present embodiment aims at stabilization of the lifting height
speed control. An automatic lifting height control is effected with
the computer controlled device shown in FIG. 15.
In such an automatic lifting height control, if the actual lifting
height speed is too quick as compared with the speed required for
suitable lifting height speed control, a control signal in the
direction of closing the first control valve 342 is fed to the
first control circuit 262 from the microcomputer 230. As a result,
if the actual lifting height speed sensed by the lifting height
sensor 102 is still quick, the microcomputer 230 delivers a control
signal in the direction of closing the first control valve 342.
However, since the change of the speed with respect to the valve
opening angle is very abrupt as shown in a characteristic curve of
FIG. 24, if, for instance, the command of the valve opening angle
.theta..sub.2 is produced, the fork 18 is stopped. If the fork 18
is stopped, the lifting speed is too slow, the microcomputer 230
produces an accelerating command (in the direction of opening the
valve). However, if the lifting speed becomes too quick in a short
time, the same operation will be caused, with the result that the
change of the speed cannot be smoothly effected and it is difficult
to stabilize the lifting speed.
The feature of the present embodiment resides in that, when
effecting a predetermined lifting height speed control, the upper
and lower limits are set to the servo valve opening command so that
the valve opening angle command is within the predetermined range,
and so that a control signal for the servo valve opening angle
command, which feeds the servomotor driving circuit 322' for
controlling the first control valve 342, is provided to the first
control circuit 262 in such a manner that the valve opening angle
command is limited to the predetermined region, is rendered to the
microcomputer 230.
In the automatic lifting height control according to this
embodiment, for instance, when effecting a low or very slow
control, the device is designed so that a speed command can be
produced solely between .theta..sub.min. and .theta..sub.max. in
terms of the valve opening angle command, as shown in FIG. 24. In
FIG. 24, the valve opening angle is equally divided into multiple
steps as indicated by .theta..sub.0 to .theta..sub.50. (For
instance, the valve opening angle may thus be divided into 50
steps).
The present embodiment of automatic lifting height control of the
present invention will be described with reference to a flow chart
for a lifting height speed control routine shown in FIG. 26 and a
characteristic curve illustrating a valve opening angle (lift valve
opening angle) versus lifting height speed shown in FIG. 25. In
FIG. 25, the valve opening angle is divided into multiple steps,
thereby making it easy to adjust the speed by increasing or
decreasing by each one pitch. In FIG. 25, the control region of
medium speed overlaps that of slow speed. The microcomputer 230
executes a lifting height speed control routine in FIG. 26. The
microcomputer 230 judges whether the speed is medium or very slow
at the step S.sub.1. If the control is placed in the medium speed
control condition, the upper limit .theta..sub.Lmin. and the lower
limit .theta..sub.Lmax. of the valve opening angle (lift valve
opening valve) is substituted for the upper limit S.sub.max. and
the lower limit S.sub.min. of the lifting height speed as shown at
the step S.sub.2. Thus, the operational speed control region is
set. In connection with the very slow control, the same setting is
effected at the step S.sub.3. The comparison between the setting
speed and the actual speed (the speed sensed by the lifting height
sensor 102) is effected at the step S.sub.4. At the step S.sub.5,
it is determined whether an increasing or decreasing of the speed
is required. When it is necessary to increase the speed, the valve
opening angle is increased by one step. At the step S.sub.6, the
judgement is made whether the speed is above the upper limit
S.sub.max. if one step is added to the present opening angle. If
the resulting speed is above the upper limit S.sub.max., one step
is not added to the present opening angle to maintain the present
opening angle of the valve (see step S.sub.9). If the speed is not
above the upper limit S.sub.max., the speed control signal added to
the present opening angle by one step is produced (see step
S.sub.8). When the deceleration of the speed is required at the
step S.sub.5, the valve opening angle is reduced by one step. At
step S.sub.7, the judgment is made as to whether the speed set to
be reduced by one step with respect to the present opening angle is
below the lower limit S.sub.min. If the resulting speed is below
the speed limit S.sub.min., the speed control signal of the present
angle of the valve is maintained (see step S.sub.9). If the speed
is above the speed lower limit S.sub. min., the speed control
signal is reduced by one step with respect to the present opening
angle (see step S.sub.10). At the step S.sub.5, if the setting
speed is equal to the actual speed, the present opening angle
command output of the valve is maintained.
Thus, the speed control command signal corresponding to any of the
steps S.sub.8, S.sub.9, and S.sub.10 in the flow chart of FIG. 26
is delivered to the first control circuit 230 from the
microcomputer 230 to effect a speed control due to the automatic
lifting height control.
The program for speed control is stored in the microcomputer 230 as
follows: When effecting medium speed control, the upper and lower
limit .theta..sub.Lmax. and .theta..sub.Lmin. of the valve opening
angle (the opening angle of the lift valve 342) corresponding to
the upper and lower limits S.sub.max. and S.sub.min. of the speed
are previously set. The microcomputer 230 delivers a speed control
command signal to the first control circuit 262 in accordance with
the flow chart shown in FIG. 26 so that the valve opening angle
lies within the above mentioned valve opening angle region. In
connection with the slow speed control, the same control is
effected.
As is clear from the foregoing, since the prior art fork lift valve
control device does not set the opening region of the lift valve in
the adjustment of the speed, the speed is too quick or slow with
the speed being beyond the predetermined region. As a result, it is
difficult to adjust the speed with the result that the speed
becomes unstable. On the contrary, according to the present
embodiment, the lift valve adjusting region is limited to the
predetermined region. Accordingly, the variable region of the
actual lifting height speed is narrowed in accordance with the
limitation of the lift valve adjusting region. As a result, the
last mentioned embodiment makes it possible to stabilize the
lifting speed.
Although several preferred embodiments of the present invention
have been illustrated and described, it is believed evident to
those skilled in the art that many changes and variations may be
made without departing from the spirit and scope of the present
invention. Accordingly, the present invention is to be considered
as limited by the following claims.
* * * * *